Unless otherwise indicated herein, the description in this section is not prior art to the claims in this application and is not admitted to be prior art by inclusion in this section.
A typical light scanning device used for an image forming apparatus such as a color printer includes a plurality of light sources, a polygon mirror, and a scanning lens. The polygon mirror reflects a light beam emitted by these light sources to scan on a circumference surface of a photoreceptor drum. The scanning lens forms images of the light beam on the circumference surface. A light scanning device, in which downsize and cost saving are desired, may be constituted of one polygon mirror that is shared among light beams of four colors of Y, C, M, and Bk, or one scanning lens that constitutes scanning optical systems of the respective colors. A four-color-sharing type light scanning device employs an opposite scanning method where a light source and an incidence optical system of two colors are located at both sides across a polygon mirror. In this case, the light beams of two colors enter one deflection surface of the polygon mirror, and the light beams of the remaining two colors enter another deflection surface.
As a light scanning device of the opposite scanning method, there is proposed a light scanning device that arranges light sources, incidence optical systems, and scanning lenses of every two colors completely symmetrically across a polygon mirror. This light scanning device has the equal angle between optical axes of light beams entering a deflection surface of the polygon mirror and respective optical axes of the light beams toward a scanned surface from this deflection surface (hereinafter referred to as an “incidence opening angle” in the disclosure) in optical paths of all the four colors. However, employing this configuration requires arranging the incidence optical systems of two colors superimposed in the sub-scanning direction at the identical position. This causes a problem that significantly reduces flexibility of an arrangement of optical components. As a technique to solve this problem, there is proposed a light scanning device that arranges, in each of the oppositely placed incidence optical systems of every two colors, the incidence optical systems of one color of two colors and the other color such that the incidence opening angles are different.